[Haskell-cafe] Problem on exception and laziness

Magicloud Magiclouds magicloud.magiclouds at gmail.com
Tue Apr 23 06:49:43 UTC 2019

Thanks. I thought $! could help on this, so tried to avoid DeepSeq. Let me try.

On Tue, Apr 23, 2019 at 2:06 PM Brandon Allbery <allbery.b at gmail.com> wrote:
> At a guess, include the parseStat in the wrap2Maybe so that the input is evaluated. Alternately, force the spine of the list produced by readFile so that the entire file is read immediately.
> Note that the error is from getContents, which produces a lazy list that reads "in the background" as the resulting list is consumed (see unsafeInterleaveIO): "files" in /proc are generated as they are read, so if a process exits while you have an open handle on one of its files in /proc, it will be invalidated on you like that.
> On Tue, Apr 23, 2019 at 2:01 AM Magicloud Magiclouds <magicloud.magiclouds at gmail.com> wrote:
>> Hi,
>> I am aware that laziness sometimes may cause exception handling in
>> unexpected ways. Recently I have such a problem that I cannot figure
>> out how to fix (simply).
>> I have a function `watch` that parse the content of /proc/. And I use
>> `catch` to wrap `readFile` call, since the data in /proc/ could be
>> gone any time.
>> ```Haskell
>> wrap2Maybe :: IO a -> IO (Maybe a)
>> wrap2Maybe f = catch ((<$>) Just $! f) (\(_ :: IOException) -> return Nothing)
>> watch args = do
>>   mStat <- wrap2Maybe (readFile ("/proc/" </> fp </> "stat")) >>=
>>     return . flip (>>=) parseStat
>>   print mStat
>>   watch args
>> ```
>> But occasionally, my tool exits with message:
>> GoM: /proc/1484/stat: hGetContents: does not exist (No such process)
>> Since above code is the only `readFile` call which leads to
>> hGetContents, I could not figure out what else should I do to catch
>> the error.
>> --
>> 竹密岂妨流水过
>> 山高哪阻野云飞
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> --
> brandon s allbery kf8nh
> allbery.b at gmail.com


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